Permanent magnets and magnetic materials based on iron, neodymium (and/or praseodymium) and boron are used worldwide in commercial applications, U.S. Pat. Nos. 5,110,374, 4,851,058 and 4,802,931 to Croat, for example, disclose a broad range of compositions that characterize the iron-neodymium-boron permanent magnet family. As indicated in these patents and in other publications, the magnets contain a transition metal (TM) component, usually iron or iron mixed with cobalt; a rare earth element (RE) component, usually neodymium including mixtures of neodymium with praseodymium and small amounts of the other rare earth group elements; and boron. As normally employed in commercial use, these compositions usually consist essentially, on an atomic percentage basis, of about 10 to 18 percent of the rare earth constituent, at least 60 percent of which is neodymium and/or praseodymium, a small amount up to about 10 percent boron, and the balance mainly iron or iron and cobalt. Preferably, these magnet compositions contain 70 percent or more of iron or iron and cobalt. The compositions may also contain small amounts of additives for processing or for the improvement of magnetic properties. They contain the tetragonal crystal phase RE.sub.2 TM.sub.14 B where RE and TM are as indicated above and below.
Sintered versions of these magnetic materials have received wide commercial acceptance. Sintered magnets are made by preparing a crystalline powder or particles containing a grain of the tetragonal crystal phase RE.sub.2 TM.sub.14 B a where RE is principally neodymium and/or praseodymium and TM is generally iron or iron and cobalt. The grains are typically one micrometer or larger such that the powder can be magnetically aligned, compacted into a green compact and sintered in vacuum or a nonoxidizing atmosphere. Sintering produces a fully dense body having magnetic coercivity. Such sintered permanent magnet is characterized by relatively large grains (i.e. greater than a few .mu.m in diameter) of the 2-14-1 phase with an intergranular phase of a rare earth element content greater than the 2-14-1 phase.
U.S. Pat. Nos. 4,981,532 and 5,110,374 (Takeshita et al) disclose a practice of treating an ingot or a powder of large grained, polycrystalline material that includes the RE.sub.2 Fe.sub.14 B phase. In the treatment, hydrogen is introduced into the polycrystalline material to form a the hydride(s). Subsequently, the hydride is decomposed and the hydrogen removed (desorbed) in older to recrystallize the 2-14-1 grain structure. In accordance with this practice, is possible to form a powder that is either magnetically isotropic or magnetically anisotropic. Thus, one starts with a material that is crystalline, contains grains of appreciable size (&gt;1 .mu.m) of the essential 2-14-1 phase and recrystallizes the grains so as to form usually smaller grains which may be aligned so as to constitute a magnetically anisotropic material. There is also a substantial market for permanent magnet compositions of fine grain structure (&lt;500 nm in average largest dimension) prepared starting with a melt spinning or other suitable rapid solidification process. The resultant powder can be used to make magnetically isotropic, resin-bonded magnets, as well as hot pressed and hot worked magnets.
The manufacture of rapidly solidified versions of the RE-TM-B family of permanent magnets starts with a molten alloy of suitable composition and produces melt-spun ribbon particle fragments. The rapid is solidification practice is usually carried out by containing the molten alloy in a heated vessel under a suitable nonoxidizing atmosphere. The molten alloy is ejected in a very fine stream from the bottom of the vessel through a small orifice onto the peripheral surface of a spinning, cooled quench wheel. The quench wheel is usually made of a suitable high-conductivity copper alloy and may have a wear-resistant coating on the circumferential quench surface of the wheel. The wheel is typically water cooled so that prolonged melt spinning production runs may be carried out without any unwanted decrease in the rate of heat extraction from the molten alloy that impinges upon the wheel. It is necessary to maintain a suitably high heat extraction rate in order to consistently obtain the desired very fine grain microstructure.
The rate of cooling of the molten alloy is dependent upon a number of factors such as the amount of superheat in the molten alloy, the temperature of the quench wheel, the rate of flow of the molten alloy through the orifice onto the spinning wheel, and the velocity of the peripheral surface of the spinning wheel. All other factors being considered, the most readily controlled parameter of the cooling of the molten alloy is the velocity of the peripheral surface of the quench wheel.
In the melt spinning of a specific composition, it is possible to obtain a range of permanent magnet properties in the melt-spun material by varying quench wheel speed. The phenomenon is well disclosed and described in U.S. Pat. Nos. 4,802,931, 4,851,058 and 5,056,585. As disclosed in these patents, by employing a given RE-TM-B composition and employing successively increasing quench wheel speeds starting with a relatively slow speed, it is possible to obtain a series of fine grained crystalline products that respectively display values of magnetic coercivity that continually increase toward a maximum value and then decrease from that value. At the same time the values of magnetic coercivity are increasing, the values of magnetic remanence also increase over at least a part of the increasing wheel speed range as the cooling rate is increased. In the manufacture of many members of the family of rapidly solidified RE-TM-B magnets, it is preferred to operate the quench wheel rate slightly faster than the wheel speed at which maximum coercivity is obtained in the melt-spun ribbon. These materials are then extremely fine grained or even apparently amorphous, and they can be annealed or hot worked to a condition of desired high coercivity and magnetic remanence.
Such melt-spun materials are magnetically isotropic. It would be advantageous to have a practice for the treatment of such extremely fine grained or amorphous materials which would produce magnetic anisotropy in such melt-spun ribbon particles. It has been possible in the prior art to produce magnetically anisotropic powder from a melt-spun ribbon material by producing overquenched, melt-spun ribbon, hot pressing the ribbon particles into a fully densified body, hot working the body to form elongated grains of magnetically anisotropic material, and pulverizing or comminuting the hot worked body to form the magnetically anisotropic powder. Such anisotropic powder has very good permanent magnet properties. However, it would be desirable to be able to produce a magnetically anisotropic material directly from (or in) the melt-spun ribbon particles.
Accordingly, it is an object of the present invention to provide a method of producing magnetically anisotropic powder material from a melt-spun powder that is initially very fine grained (typically less than 50 nanometers in grain size) or even apparently amorphous in its microstructure. It is a more specific object of the present invention to introduce such magnetically anisotropic properties into a melt-spun material by a practice of absorbing hydrogen into the fine grained material and then removing the hydrogen under conditions which produce a fine grain material having anisotropic magnetic properties.
In accordance with a preferred embodiment of our invention, these and other advantages are accomplished as follows.